1. Field of the Invention
The invention relates to light emitting diode (LED) based signal lamps and in particular to systems in which a phosphor, photo luminescent material, is used to generate a desired color of light. Moreover, the invention concerns LED signal lamps or light modules for traffic lights and signal lights.
2. Description of the Related Art
Traffic lights, also known as traffic signals, stop lights etc. for vehicles and pedestrians are well known and comprise red and green signal lamps in which red denotes stop and green (sometimes white for pedestrian walk symbols) denotes go. Often vehicle traffic signals include an amber signal lamp to indicate to prepare to stop. Signal lamps generally comprise an open housing/casing containing a light source, traditionally an incandescent light bulb, and a front tinted convex cover lens which is in the form of a colored filter. The front cover lens is often fabricated from a hard abrasion resistant plastics material with a lens structure formed on its inner surface to act as an optical condenser with the filament of the lamp placed at the focal point of the optical condenser such that the lamp projects light to a focal point at infinity. Such lamps produce very high intensity light within a standardized narrow solid angle enabling them to be observed at a distance even in bright ambient light. The front cover which is generally convex in shape is often tinted to reduce glare and the reflection of sun light. The different signal colors/hue for automotive, aviation, rail, nautical and other applications are specified by various government agencies and trade organizations in terms of their x and y chromaticity coordinates on the CIE (Commission Internationale d'Eclairage) chromaticity diagram. For example in the USA the Institute of Transportation Engineers (ITE) specifies the color specifications for vehicle and pedestrian traffic signals, the Federal Aviation Administration (FAA) specifies aviation ground light colors, the International Civil Aviation Organization (ICAO) specifies aeronautical ground light colors, the Engineering society for advancing mobility land sea air and space (SAE) specifies ground vehicle lighting color standards and the American Railway Engineering and Maintenance-of-way Association (AREMA) specifies color signal specifications for railroad applications.
The development of high intensity LEDs having lower power consumption, lower heat generation and longer operating lives compared to incandescent sources has led to a new generation of LED based signal lamps. Currently, LED signal lamps utilize an array of color LEDs. The LED array can contain many hundreds of LEDs, typically 200-600 standard intensity (e.g. 40 to 120 mW) LEDs distributed over the entire surface of the lamp module or an array of 18 to 24 high intensity (e.g. 1 W), flux, LEDs concentrated about the central axis of the lamp module. For example InGaN, GaAlAs and AlInGaP based LEDs are respectively used to generate red (610 nm), green (507 nm) and amber (590 nm) light. In such systems the front cover lens is often tinted or incorporates a complimentary color filter.
A problem with LED based traffic signals is thermal stability. For example the intensity of light output of an AlInGaP amber LED will drop nearly 75% over an operating temperature range of 20 to 80° C. Although red and green LEDs have a relatively lower drop off in intensity, the wavelength (color) changes with temperature. As a result LED signal lamps will often incorporate a feedback circuit to minimize their wavelength temperature dependency.
A further problem with LED based traffic signals is that a failure of one or more of the LEDs can lead to problems of intensity uniformity across the lamp surface. U.S. Pat. No. 5,947,587 teaches using a Fresnel lens as a spreading window for an LED signal lamp to provide an optimum, homogeneous brightness distribution of output light. The Fresnel lens is positioned between the LED array and an outer cover. The LEDs are clustered around the axis of the lamp to ensure that failure of one or more LEDs has little or no effect on the output light.
Conversely, US 2007/0091601 describes an LED traffic light structure having an array of LEDs which are spread over substantially the entire light emitting surface area of the lamp. A front cover which comprises multiple rectangular lenses is provided over the LEDs and an inner cover sandwiched between the front cover and the LEDs and comprising columns symmetrically arranged relative to a central axis on an emergence surface of the inner cover. Light scattered and reflected by the inner cover is inclined downwards to a horizontal axis of the front cover to thereby reduce color difference in the emitted light.
US 2006/0262532 concerns an optical condenser for use in an LED signal lamp. The LEDs are provided as an array on a base plate and the lamp configured such as to deliberately de-focus the emitted light. De-focusing can be achieved by locating the LEDs at the focal plane of the condenser and the condenser has a configuration of optical structures, such as spherical lenses, to de-focus the light. Alternatively the LED array, base plate, is located slightly away from the focal plane of the optical condenser.
For pedestrian crossing signals, such as ones in which a white pedestrian walk symbol and red raised hand symbol denote “walk” or “cross” and “wait” or “do not cross” respectively, the “wait” symbol can be operational virtually twenty four hours a day seven days a week and in hot climates it is found that the red LEDs used to generate the symbol can have thermal stability problems and have to be replaced. Secondly, since the symbols are generated by an array of LEDs configured in the form of the required symbol, failure of one or more LEDs leads to an appreciable degradation of the symbol's appearance.